In the current High-Speed Downlink Packet Access (HSDPA) feature of UMTS, mobile stations (MSs) make periodic measurements of the received downlink channel quality. These measurements are reported to the serving base station (BS) in the form of indications of a modulation and coding scheme (MCS) that the MS estimates it can decode with a certain probability under the measured channel conditions. These measurement reports are known in UMTS as Channel Quality Indicators (CQI). In each sub-frame, the serving BS uses the CQI reports received from different MSs to select a subset of the MSs (typically those with better channel conditions) to receive a data packet. Each MS receives and decodes one or more control channels (High-Speed Shared Control Channel, HS-SCCH) in each sub-frame in order to ascertain whether the BS has scheduled it to receive a data packet.
In order to simplify the processing in the MS, it is specified in UMTS that packets transmitted to a single MS in consecutive sub-frames must be signalled using the same HS-SCCH. However, in cases when no packet was transmitted to the same MS in the immediately-preceding sub-frame, the MS may be required to monitor up to four HS-SCCHs in order to give sufficient flexibility to the BS's scheduler.
Receiving and decoding the HS-SCCH control channels consumes energy in the receiver and processor of the MS. In order to increase the battery life of the MS, it is desirable to switch off the MS's receiver for as much as possible of the time when data packets are not being received.
One known way of achieving this is to move the MS to a state where it is not required to monitor the HS-SCCHs. The MS can be moved to this state by means of a command transmitted by the BS. However, a relatively long signalling process is then required to move the MS back to a state where it can receive data packets, which increases the delay for packet transmission and can have an adverse effect on user experience.
Another known procedure is to re-configure the operation of the MS in order to reduce the number of sub-frames in which the MS is required to monitor the HS-SCCHs. For example, by means of a command transmitted by the BS, the MS could be instructed to monitor only every nth sub-frame so that it can switch off part or all of its receiver and processor in the other sub-frames. However, this restriction reduces the effectiveness of the scheduling algorithm employed by the BS, as the BS has less freedom to transmit data packets on occasions when the channel conditions to that MS are most suitable.